Emulsions have been desirable vehicles for the manufacture and transportating of synthetic polymeric flocculants, particularly high molecular weight polymers. Among other reasons for their popularity, emulsion polymers can be prepared with higher polymer solids and provide substantial cost savings over previous solution polymers. Microemulsions, as taught in U.S. Pat. Nos. 4,956,399; 4,956,400; 5,037,863; 5,132,023 and 5,037,881, provide additional advantages with respect to polymers exhibiting undesirable characteristics even in the context of emulsions, for example Mannich (alk)acrylamide polymeric flocculating agents, by providing for high solids level, reduced debilitating crosslinking and superior performance. Though many polymers are commercially available in powder form, this powder form creates dust problems and the process of dissolving the dry solids, in aqueous medium, is a time-consuming step.
Despite the aforesaid advantages of using emulsion polymers over solution and powder polymers, as a practical matter, emulsions and microemulsions are not universally used because they can exhibit stability problems, settling tendencies and high bulk viscosity which can make handling difficult and costly. These problems were purportedly mitigated in U.S. Pat. Nos. 4,619,967 and 4,565,836 which disclose inverse emulsions containing a single water-soluble polymer in an aqueous phase having two distinct aqueous droplet size distributions. The process in which the patentees teach to prepare these stable emulsions, however, involves an arduous series of steps of applying different shear rates to particular portions of the emulsion to produce two different aqueous droplet size distributions.
The inventors of the instant invention have surprisingly discovered a much simpler, more efficient process for making stable inverse emulsions having two or more different aqueous droplet size distributions or modes. In addition to providing a bimodal or multimodal emulsion having high solids with low bulk viscosity and, with certain polymers, superior flocculation performance, the process of the present invention provides for manufacture simplification and flexibility, allows for greater control over the dispersed phase or aqueous droplet sizes and can be used to prepare emulsion blends containing two or more different polymers. This last advantage is particularly important in view of the known benefits of combining two different water-soluble polymers for water treatment applications.
Japanese patent nos. 20-09500 and 63-218246 disclose the mixing of an inverse emulsion containing a cationic polymer and an inverse emulsion containing an anionic polymer. The resulting emulsion mixtures are described as providing improvements in flocculation performance and benefits in paper making, respectively. But there is no teaching to mix two or more different types of emulsions, one of which is a microemulsion, to produce the desired multimodal emulsion having lower bulk viscosity and higher solids as achieved by the instant invention.
In U.S. Pat. No. 5,213,693 the performance and handling benefits of simultaneously treating waste water with a cationic coagulant polymer and a cationic flocculant polymer are described. There, a particulate mixture containing coagulant polymer beads and flocculant polymer beads is used to facilitate dewatering of a sludge suspension. The beads generally range from about 70 to 1000 microns in size and are made by reverse phase suspension polymerization followed by drying and recovering the dry beads from the liquid. While the patentee mentions that the particulate composition can be a reverse phase emulsion, or more preferably, a reverse phase "substantially dry" dispersion containing the two polymers, there is no mention of using a microemulsion and no teaching to combine two or more inverse emulsions having polymer-containing aqueous phases which differ in their aqueous droplet size distribution. The aqueous droplets in the patentee's emulsions or dispersions are merely disclosed as ranging in size of up to 10 microns. No improvements in the physical properties of the emulsion are even suggested. In contrast, the process of the present invention blends at least two emulsions having aqueous droplet size distributions with different average droplet sizes, one of which results from the microemulsion. Apparently, the different droplet sizes in the microemulsion and second emulsion used in the process of this invention are retained in the resulting emulsion. These resulting bimodal and multimodal emulsions (collectively called "multimodal emulsions") exhibit lower viscosity. The smaller droplets from the microemulsion which are retained in the final multimodal emulsion are particularly beneficial for employing water-swellable or water-soluble polymers which tend to crosslink, such as the Mannich (alk)acrylamide polymers disclosed in, for example, U.S. Pat. No. 4,956,399; in such instances, the debilitating effects of large-scale crosslinking is minimized by the smaller droplets within the bimodal emulsion.
U.S. Pat. No. 4,916,182 ('182 patent) discloses the blending of a water-in-oil emulsion containing a water soluble anionic polymer with a water-in-oil emulsion containing a water soluble cationic polymer to form an emulsion mixture which is used as an adhesive composition for wall covering. After the two emulsions are blended, the resulting emulsion is subject to high shear to create the desired particular size range of about from 2 to 5 microns. There is no teaching to blend two emulsions having distinctly different aqueous droplet size distributions, nor is it suggested that a microemulsion be used to prepare a multimodal emulsion. The emulsion mixtures produced according to the '812 patent do not provide the benefits of the present invention and the benefits of the present invention are not described in the '812 patent.
As described in RubberWorld, 138, 877(1958), multimodal systems having at least two different particle size distributions were observed as providing reduced viscosity. Latices having average particle diameters of 950 Angstroms (.ANG.), 1710 .ANG. and 3250 .ANG. were concentrated alone and in various blend ratios of small, medium and large particles. These water-insoluble styrene-butadiene latices are, however, very different from the emulsion polymers of the present invention, having different applications and posing different problems and, having an aqueous continuous phase, they are not inverse emulsions.
Similarly, U.S. Pat. No. 4,456,726 discloses a method for making a concentrated, bimodal synthetic resin dispersion which lacks structural viscosity. Such resins must be water insoluble under the conditions of preparation and use and the resin dispersions have an aqueous continuous phase; thus they are completely different from the polymers used in the present invention.
U.S. Pat. No. 5,100,951 (the '951 patent) teaches that inverse emulsions containing high molecular weight cationic polymers can be combined with aqueous solutions of lower molecular weight cationic polymers. A different concept of producing the combination of polymers is disclosed in columns 9 and 10 of that patent, which involves emulsifying the solution polymer by adding oil and surfactant and applying intense mechanical agitation. The emulsified liquid polymer is then blended with a commercial emulsion polymer, which also requires intense mechanical agitation. Such intense agitation is not required in the present invention in order to produce a stable multimodal emulsion. The '951 patent also fails to suggest the blending of two or more emulsions having different droplet size distributions, such as a microemulsion and a macroemulsion. Bimodal emulsions are not disclosed on the '951 patent. The patentee in the '951 patent teaches to add additional oil and surfactants to the emulsion polymer before mixing the liquid polymer into the emulsion to achieve a stable blend. The use of additional oil and surfactants, which greatly increases costs, is also avoided in the process of the present invention.
In comparison to known methods, this invention provides a convenient, flexible process for preparing a low viscosity water-in-oil emulsion which can comprise more than one type of polymer. For example, two polymers having two different ionic charges may be combined to attain a system having a desired intermediate charge. Accordingly, the inverse emulsions produced by the process of the instant invention are superior to emulsions in the art because they not only exhibit low bulk viscosity and high solids content, but they can accommodate more than one type of polymer.
Another advantage of the present invention is that the properties of the multimodal emulsion blends can be easily adjusted by simply changing the particular ratios in which the polymeric microemulsion and second emulsion are mixed, or by changing the ratios of microemulsion polymer to the polymer in the second emulsion. This is particularly desirable from a commercial standpoint because it allows for versatility in forming the particular bimodal or multimodal emulsion that is appropriate for treating a particular type of aqueous dispersion. One can, for example, tailor the ratio of the two emulsions that are blended to meet specific requirements and then simply mix the two emulsions accordingly to obtain the optimal bimodal emulsion; this is much simpler than processes used in the art.
In yet another aspect of the present invention, a stable multimodal emulsion exhibiting superior flocculation performance is prepared using a microemulsion comprising a water-soluble polymer-based polymer having functional groups which are capable of continually crosslinking. More preferably, the polymer in the microemulsion is a quaternary dimethyl aminomethyl (alk)acrylamide which is capable of crosslinking at ambient conditions. This microemulsion is blended with a second emulsion, preferably a macroemulsion, containing a polymer which is preferably cationic and a blend stabilizing amount of aldehyde scavenger.
The preferred stable multimodal emulsion blends produced by the present invention exhibit flocculation performance that is just as effective, if not more effective, than a single microemulsion or macroemulsion containing a polymer of identical charge.